1. Field of the Invention
The present invention relates to a liquid crystal display device as a computer monitor and a liquid crystal display device for displaying video images and, more particularly, to a liquid crystal display device having desirable viewing angle characteristics.
2. Description of the Related Art
Various display modes have been proposed for improving the viewing angle of a liquid crystal display device. Typical examples of such display modes include: {circle around (1)} IPS (In-Plane Switching) mode which uses a transverse electric field to move the liquid-crystal molecules in parallel with the substrate surface; {circle around (2)} a liquid crystal display device in which the liquid crystal molecules are oriented substantially vertically to the substrate surface in the absence of an applied voltage, and the liquid crystal molecules in different regions are inclined in respectively different directions in the presence of an applied voltage (Japanese Laid-Open Publication No. 7-28068); {circle around (3)} a liquid crystal display device in which the liquid crystal molecules are oriented substantially horizontally to the substrate surface in the absence of an applied voltage, and the liquid crystal molecules in different regions rise in respectively different directions in the presence of an applied voltage, thereby improving the viewing angle of the device (Japanese Laid-Open Publication No. 10-3081); and {circle around (4)} a liquid crystal display device of a normally black mode producing a black display in the absence of an applied voltage, in which an optical compensator is used to improve the viewing angle of the device, (Japanese Laid-Open Publication No. 5-289097).
In the IPS mode {circle around (1)}, however, it is necessary to provide a plurality of opaque electrodes in a pixel, thereby reducing the aperture ratio and thus the transmissivity (brightness) of the device. The liquid crystal display device {circle around (2)} disclosed in Japanese Laid-Open Publication No. 7-28068 employs a liquid crystal material having a negative dielectric anisotropy (n-type liquid crystal material) and a substrate which has been subjected to a vertical alignment treatment. In such a case, the liquid crystal injection process takes a period of time twice as long as that for a device which employs a p-type liquid crystal material having a positive dielectric anisotropy and a substrate which has been subjected to a horizontal alignment process, thereby reducing the production efficiency. The liquid crystal display device {circle around (3)} disclosed in Japanese Laid-Open Publication No. 10-3081 employs transparent electrodes provided respectively on the upper and lower substrates to drive the liquid crystal molecules, thereby avoiding the reduction in transmissivity as in the IPS mode. Moreover, The liquid crystal display device {circle around (3)} employs a liquid crystal material having a positive dielectric anisotropy and a substrate which has been subjected to a horizontal alignment treatment, thereby also avoiding the reduction in production efficiency as in the device {circle around (2)}. However, the device {circle around (3)} has viewing angle characteristics that are inferior to those of the device {circle around (2)}. Particularly, the device {circle around (3)} has asymmetric gray scale characteristics in the vertical direction along the display plane.
Referring to FIG. 55, the liquid crystal display device {circle around (4)} disclosed in Japanese Laid-Open Publication No. 5-289097 includes a liquid crystal panel 4, a birefringence anisotropy compensation panel 3 provided optically continuously on the liquid crystal panel 4 for optically compensating the birefringence anisotropy of the liquid crystal panel 4 along the plane thereof, a viewing angle dependency compensation panel 2 provided on the birefringence anisotropy compensation panel 3, and a pair of polarizers 1 and 5 interposing the panels 2, 3 and 4 therebetween, so that the absorption axes (1.1) and (5.1) are perpendicular to each other. The birefringence anisotropy compensation panel 3 is arranged so that the optical axis (3.1) or (3.2) (or rubbing direction) thereof is parallel to the substrate surface of the liquid crystal panel 4 and perpendicular to the optical axis (4.1) or (4.2) (or rubbing direction) of the liquid crystal panel 4. The viewing angle dependency compensation panel 2 is arranged so that the optical axis (2.1) (or rubbing direction) thereof is perpendicular to the substrate surface of the liquid crystal panel 4. The device {circle around (4)} provides a certain level of improvement in the viewing angle characteristics thereof by employing the viewing angle dependency compensation panel 2. However, the contrast of the device {circle around (4)} is reduced as the viewing angle is inclined or shifted from a direction normal to the display plane. Thus, the viewing angle characteristics of the device {circle around (4)} are not sufficiently desirable. Moreover, it is difficult to stably obtain a uniform orientation and transmissivity across the display plane of the liquid crystal panel 4 of the device {circle around (4)} in the presence of an applied voltage.